Download Seedling emergence patterns

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Seedling emergence patterns
Learning objective: To be able to recognize the two major seedling emergence patterns.
Background information: There are two common patterns of seedling emergence. The
most common pattern is where the cotyledons are raised above ground as the seedling
emerges. The seedling forms a hypocotyl hook that pushes through the soil. Once the
seedling reaches the light, the hook opens to create a straight seedling. This pattern is
called epigeous.
In the second pattern, the cotyledons remain underground and the stem (epicotyl)
emerges above ground. This pattern is called hypogeous. Large seeded plants like
Kentucky coffeetree and oaks tend to have hypogeous germination.
The cotyledons function as the repository for food reserves in the seed. During
seed development the plant stores food in the cotyledons that the seedling will use for
emergence. Once the seedling emerges into the sunlight, it can begin to make its own
food through photosynthesis.
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In the group of plants covered in this website, the oaks, nuts (Ohio buckeye, black
walnut and pecan), paw paw, and Kentucky coffeetree have a hypogeous seedling
emergence pattern. For black walnut, pecan, and paw paw, the cotyledons never emerge
from the seed. All the other plant species on the website have a epigeous seedling
emergence pattern.
An easy comparison can be made using Kentucky coffeetree and honeylocust. They are
both in the legume family and only have physical dormancy. The seeds are easy to
collect, are large and store for a very long time so the experiment can be conducted any
time of the year.
Procedures:
1. Scarify the seeds using a file. Sow seeds approximately an inch deep in pots
containing a greenhouse potting mixture. Keep in a sunny location and do not
allow the potting mixture to dry out. The seedlings should emerge within two
weeks.
2. When the seedlings reach a stage when they have true leaves, gently wash off the
potting mixture from around the roots of the seedlings. Help the students to
identify parts of the seedling and the location of the cotyledons. Notice how the
cotyledons on the honeylocust are leaf-like and green. They do carry out
photosynthesis in the light. The cotyledons on the Kentucky coffeetree remain
underground and are large and white. Observe how thick the root on the Kentucky
coffeetree has become. A lot of the food reserves in the cotyledon has been
transferred to the root even before the shoot has emerged above ground. This is
one reason it takes longer to see the shoot emerge in the Kentucky coffeetree
compared to the honeylocust.